The present invention relates to an optical transmission device or an optical communication module whose optical components have satisfactory optical coupling characteristics. The present invention can also provide optical transmission components whose optical components have satisfactory optical coupling efficiency. The present invention is well adapted to optical communication systems and optical communication networks.
To implement satisfactory transmission characteristics in an optical communication system, an optical transmission device must have a high output capability, and this requires highly efficient optical coupling between a light source, e.g., a semiconductor laser diode (LD) and a dielectric waveguide or optical fiber during the process of mounting the optical transmission device. However, there is a wide discrepancy between the beam-emerging angle of a conventional semiconductor laser device and the light-receiving angle of an optical fiber or optical waveguide, and thus the optical coupling efficiency obtained through their direct coupling is not satisfactory.
To overcome this problem, a conventional optical module for communications must use an optical lens for compensating the discrepancy and further adjust the optical axes of its laser, lens and optical fiber highly accurately during the mounting process. On the other hand, for economized utilization of optical communications, a reduction in the size, power dissipation and cost of an optical transmission device has recently been called for. Therefore, economization and mass-production of various components, especially, an optical module incorporated in an optical transmission device are essential in achieving an inexpensive optical communication system.
For these needs, to improve coupling efficiency in direct coupling and to relax alignment tolerance of optical axis, attempts have been made positively to add a lens function to a semiconductor laser device by integrating a propagating mode converter or a guided mode converter with the semiconductor laser device.
The mode converter means a component for narrowing a beam divergence of a semiconductor laser device that has wide angles of divergence ranging from, e.g., 30 to 40 degrees. The mode converter improves the optical coupling efficiency between a semiconductor laser device and an optical transmission path such as an optical fiber.
The integration of mode converters has been implemented mainly with Fabry-Perot semiconductor laser devices. However, to increase the transmission speed and capacity of an optical communication system, integration of a mode converter with a distributed feedback (DFB) semiconductor laser device is requisite. Thus, integration of a mode expanding device with a DFB semiconductor laser device has been started, some examples of which are reported in the Preliminary Transactions 3p-ZC-5, 3p-ZC-6, etc of Preliminary Transactions for the 58th Conference of the Japan Society of Applied Physics.
However, when a mode converter is integrated with a DFB semiconductor laser device as described above and thus their optical coupling efficiency is improved, the following inconvenience occurs. That is, when the coupling efficiency is improved, the light that has reflected at the incident end face of an optical transmission path, the repeating points along the optical transmission path and the like returns to the DFB semiconductor laser device easily. The returning reflected light causes noise and the like within the semiconductor laser device, thus becoming a main source of deteriorating the transmission characteristics of an optical module, an optical transmission device, and hence of a communication system. Therefore, to make the most of the excellent feature of mode converter integration, it is technically critical to establish satisfactory resistance to reflected light.
An object of the present invention is to provide an optical transmission device that can improve its resistance to returning reflected light by improving the resistance to returning reflected light of a semiconductor laser device, which is a light source, while maintaining optical coupling between the light source and an optical transmission path highly efficiently.
One aspect of the present invention provides an optical transmission member or an optical transmission device having features not disclosed in the aforementioned prior art (reported in the Preliminary Transactions 3p-ZC-5, 3p-ZC-6, etc of the 58th Conference of the Japan Society of Applied Physics), so that high optical coupling efficiency characteristics of a mode converter-integrated laser is maximized. That is, the optical transmission member or the optical transmission device provided by the invention has novel features such that a mode converter having an optical waveguide with a modulated thickness and a gain-coupled (GC) DFB semiconductor laser device are integrally formed.
The inventors paid attention to the fact that a GC DFB semiconductor laser device exhibits better resistance to returning reflected light than an ordinary DFB semiconductor laser device. The effect of laser oscillation by gain guiding to reduce returning light noise has already been verified in a self-oscillating semiconductor laser element using a Fabry-Perot resonator. In the Fabry-Perot resonator, regions having different gains arranged in a stripe pattern in the direction of the resonator exerts considerable influences on multimode laser oscillation, and these regions cancel out the returning light noise. In a semiconductor laser element used for optical transmission, self oscillation is what has to be avoided, because self oscillation, i.e., the phenomenon in which the intensity of laser oscillation fluctuates in small pulsations only by applying current independently of an input signal from outside the element, is harmful to the transmission accuracy of an optical signal. On the other hand, in a DFB semiconductor laser device in which the present invention is embodied, the regions having different gains have been utilized to concentrate large quantities of current at a predetermined location in a light-emitting region where a diffraction grating is additionally provided. Laser oscillation conditions in a resonator of this type depend greatly on the pattern of a diffraction grating that is formed along the light-emitting region.
However, according to the knowledge empirically obtained by the inventors, even if the regions having different gains are provided in a stripelike form in such a DFB semiconductor laser device, it has been found out that noise attributable to an optical beam accidentally injected from an optical fiber can be reduced. Even if a mode converter is provided between the laser oscillating region and the optical fiber, the effect is such that the aforementioned noise is almost negligible. That is, when a mode converter analogous to a beam spot expander disclosed in JP-A-9-171113 is interposed between a laser oscillating region having a DFB resonator structure and an optical fiber, a laser beam generated at the laser oscillating region can be introduced into the optical fiber efficiently. However, the aforementioned returning light injected from the optical fiber is also transmitted to the laser oscillating region efficiently. Therefore, a DFB semiconductor laser device having the beam spot expanding region recited in JP-A-9-171113 can improve the transmission efficiency of an optical signal, but may have to provide some measures to prevent the noise. Another object of the present invention is to overcome such problem over the trade-off encountered in the conventional integrated optical element.
Another aspect of the present invention provides a mode converter-integrated DFB semiconductor laser device having an excellent resistance to returning reflected light and capable of highly efficient optical coupling to an optical transmission path and thereby provides an optical transmission device having an excellent resistance to returning reflected light with high optical output. When such an optical transmission device is used, an optical transmission system having an excellent resistance to returning reflected light can be provided while maintaining optical coupling between a light-emitting portion and an optical transmission path highly efficiently. In an optical transmission device that is a specific embodiment of the present invention, a mode converter-integrated DFB semiconductor laser device is optically coupled to an optical fiber directly (without interposing any optical element therebetween).
Some features of the waveguide-type optical element, optical module, and optical transmission device of the present invention may be as follows.
First, a waveguide-type optical element includes an optical waveguide layer having a refractive index suitable for light propagation and a structure over a semiconductor substrate. The structure is sandwiched by clad layers made of a material having a wider band gap and a lower refractive index than a material of the optical waveguide layer. The optical waveguide layer has, at least in part thereof, a light-emitting region and a mode-converting region optically coupled to the light-emitting region. The light-emitting region has, at least in part thereof, a diffraction grating whose refractive index cyclically changes in a direction of travel of the light. In a region where the diffraction grating is provided, gains or losses of the light change cyclically.
It is preferable that the mode-converting region be formed so that the thickness of the optical waveguide layer continuously decreases in the direction of travel of the light. It is also preferable that the mode-converting region be formed so that the thickness of the optical waveguide layer continuously decreases in the direction of travel of the light and that the width of the optical waveguide layer transverse to the direction of travel of the light changes toward a light emerging portion.
On the other hand, it is preferable that the optical waveguide portion be of a ridge-waveguide type and that the width of the optical waveguide transverse to the direction of travel of the light be modulated in a direction of propagation of the light.
Next, an optical module has a waveguide-type optical element for emerging light and an optical transmission path that is to be optically coupled to the waveguide-type optical device. The waveguide-type optical device has an optical waveguide layer having a refractive index preferred for propagating the light. The optical waveguide layer has, at least in part thereof, a light-emitting region and a mode-converting region optically coupled to the light-emitting region. The light-emitting region has, at least in part thereof, a diffraction grating whose refractive index cyclically changes in a direction of travel of the light. In a region where the diffraction grating is provided, gains and losses of the light change cyclically.
Further, it is preferred in the optical module that an end face of the waveguide-type optical element from which the light emerges confront an end face of the optical transmission path on which the light is incident. It is also preferred that the thickness of the optical waveguide layer continuously decrease in the direction of travel of the light in the mode-converting region of the waveguide-type optical element. It is further preferred that the optical waveguide portion be of a ridge-waveguide type.
An optical transmission device has an optical module for generating an optical signal. The optical module has, at least in part thereof, a waveguide-type optical element for emerging light and an optical transmission path that is to be optically coupled to the waveguide-type optical element. The waveguide-type optical element includes an optical waveguide layer having a refractive index preferred for propagating the light. The optical waveguide layer has, at least in part thereof, a light-emitting region and a mode-converting region optically coupled to the light-emitting region. The light-emitting region has, at least in part thereof, a diffraction grating whose refractive index cyclically changes in the direction of travel of the light. In a region where the diffraction grating is provided, gains or losses of the light change cyclically.
It is preferable in the optical transmission device that an end face of the waveguide-type optical element from which the light emerges confront an end face of the optical transmission path on which the light is incident. Further, it is preferable that the thickness of a laminated body constituting the optical waveguide continuously decrease in a direction of emergence of the light in the mode-converting region of the waveguide-type optical element. It is preferable that the thickness of the optical waveguide which continuously decreases in the direction of emergence of the light do not exceed, at a light-emerging end face, ⅓ of the thickness of the optical waveguide in a light-emitting portion. Further, it is preferable in the optical transmission device that the width of its waveguide portion in a direction perpendicular to the optical axis be modulated toward a portion from which the light emerges. It is also preferable that the optical waveguide portion be of a ridge-waveguide type.
The GC DFB semiconductor laser having a resonance structure suitable for embodying the present invention means a semiconductor laser device which has a diffraction grating whose refractive index cyclically changes in the direction of travel of light of the semiconductor laser device and in which gains or losses of the light cyclically change in a region where the diffraction grating is provided. Reflection resistance of a conventional GC DFB semiconductor laser device is discussed in detail in such literature as IEEE Journal of Quantum Electronics, Vol. 27, No. 6, pp. 1732-1735.